The invention concerns a process for the selective hydrogenation in the gas phase of acetylene hydrocarbons containing 2 or 3 carbon atoms (acetylene or propyne) to the corresponding ethylenic hydrocarbons (ethylene or propylene).
Ethylene is a monomer used for the preparation of a large number of polymers. It is generally obtained by hydrocarbon pyrolysis or steam cracking processes. The ethylene produced contains small quantities of acetylene (generally less than 3%) which must be eliminated before use. The acetylene contents which are tolerated in ethylene for use in polymer production are generally less than 10 ppm, more often less than 5 ppm.
One of the techniques used to eliminate acetylene in ethylene is to selectively hydrogenate it to ethylene in the presence of a palladium based, catalyst supported on a refractory support such as alumina. The general problem with these monometallic catalysts (constituted solely by palladium supported on alumina) is that, when the operating conditions are such as to permit complete elimination of the acetylene, a portion of the ethylene is also converted to ethane. In addition, these monometallic catalysts generally have relatively low stabilities due to the formation of large amounts of oligomers which gradually cover the catalyst surface under the reaction conditions. This deposit of hydrocarbon can, of course, be eliminated by controlled oxidation processes, but it is of advantage in an industrial process to have as long a lifetime as possible for the catalyst between two regenerations.
The addition of enhancers to the palladium has frequently been proposed in order to improve catalyst properties. These additives can be, for example, silver (U.S. Pat. No. 2,802,889), and iron and silver (U.S. Pat. No. 3,243,387).
These enhancers can also be selected from alkaline or alkaline-earth metals such as lithium (U.S. Pat. No. 3,325, 556), potassium (European patent application EP-A- 0 124 744) or calcium (U.S. Pat. No. 4,329,530).
Whether using monometallic catalysts (catalysts based on palladium alone) or enhanced catalysts (catalysts containing palladium and at least one other element), the skilled person knows that when the palladium is concentrated at the surface of the spherules of the catalyst, catalytic performance is substantially superior to that of a catalyst with an identical form in which the palladium is homogeneously distributed through the spherules of the catalyst. In the case of bimetallic palladium-silver formulations, for example, it has been discovered that when the palladium is situated at the periphery of the spherules of the catalyst and that the silver is homogeneously distributed, the catalyst has improved properties (U.S. Pat. No. 4,404,124; EP-A-0 064 301 and French patent FR-A-2 597 113), in particular as regards a lower degree of ethane and oligomerisation product formation.
Further, Japanese patent application JP-A-04 108 540 describes liquid phase selective hydrogenation catalysts for 1,3-butadiene, in which silver is precipitated on and supported by a palladium surface. In these catalysts, the support consists of alumina with a relatively high specific surface area and a Ag/Pd weight ratio of 0.3 to 5.0, preferably 0.5 to 3.0.